1. Field of the Invention
The present invention relates in general to degaussing circuits, and more particularly to a degaussing circuit for removing a magnetic field upon application of a power supply and then controlling an alternating current (AC) current flowing through a positive temperature coefficient resistor to a degaussing coil so as to reduce power consumption and avoid an increase in an ambient temperature.
2. Description of the Prior Art
FIG. 1 is an illustration of a circuit diagram of a conventional degaussing circuit. As shown in this drawing, the conventional degaussing circuit comprises a rectifying circuit 1 for rectifying an AC voltage, inputted through a switch SW1 and a fuse, into a direct current (DC) voltage, a capacitor C1 for smoothing the DC voltage from the rectifying circuit 1 to maintain a constant voltage Vo at an output terminal of the degaussing circuit, a degaussing coil DE for removing a magnetic field present in a tube of a monitor or a television (TV) upon application of a power supply voltage, and a thermistor 2 for controlling the flow of a current through the degaussing coil DE to maintain a constant temperature. The thermistor 2 includes a positive temperature coefficient resistor Ra having a resistance which increases with an increase in the temperature, and an internal resistor Rb for generating heat to maintain the resistance of the positive temperature coefficient resistor Ra constant.
The operation of the conventional degaussing circuit with the above-mentioned construction will hereinafter be described.
As the switch SW1 is turned on, the AC voltage is applied through the fuse to the rectifying circuit 1, which rectifies the applied AC voltage into the DC voltage. The DC voltage from the rectifying circuit 1 is smoothed by the capacitor C1, thereby causing the voltage Vo at the output terminal to be maintained constant.
On the other hand, if the AC current flows to the degaussing coil DE through the positive temperature coefficient resistor Ra in the thermistor 2 upon application of a power supply voltage, the magnetic field in the tube of the monitor or the TV is removed by the degaussing coil DE.
The resistance of the positive temperature coefficient resistor Ra in the thermistor 2 is noticeably smaller than 20.OMEGA. at a normal temperature of, for example, 25.degree. C. As a result, at the normal temperature of 25.degree. C., the AC current flows to the degaussing coil DE through the positive temperature coefficient resistor Ra upon application of a power supply voltage. However, after a predetermined time period (for example, 1 to 2 sec) has elapsed, the resistance of the positive temperature coefficient resistor Ra rises to several tens mega ohms M.OMEGA. as the temperature rises due to the AC current. In this case, the positive temperature coefficient resistor Ra is as good as opened.
At this time, the internal resistor Rb in the thermistor 2 generates the heat so that the temperature of the positive temperature coefficient resistor Ra can be maintained constant. As a result, the resistance of the positive temperature coefficient resistor Ra remains at a large value. Hence, the AC current flows to the degaussing coil DE only upon application of a power supply voltage, whereas it does not flow to the degaussing coil due to the resistance of the positive temperature coefficient resistor Ra after the lapse of the predetermined time period.
However, the above-mentioned conventional degaussing circuit is disadvantageous since the internal resistor Rb generates the heat continuously to maintain the temperature of the positive temperature coefficient resistor Ra constant, which results, in large power consumption and an ambient temperature increase.